Comprehensive RAD Lab and Radiologic Physics Exam Study Guide

Fundamental Principles of Radiographic Technique: kVp and mAs

Kilovolt peak, commonly referred to as kVp, is the primary controller of radiographic contrast. It should be understood that kVp governs the quality, energy, and penetrability of the x-ray beam, but it does NOT describe or control the quantity of x-rays; quantity is managed by milliampere-seconds ($mAs$). A higher kVp results in a lower contrast image, which is described as a long-scale contrast involving many shades of gray. The "p" in kVp specifically stands for peak. One of the critical operating guidelines for kVp is the $15\%$ rule, which states that a $15\%$ increase in kVp will approximately double the receptor exposure, while a $15\%$ decrease will halve it. This rule is frequently utilized to adjust technique while affecting image contrast.

milliampere-seconds ($mAs$) serves as the primary controller of radiographic density, which is defined as the overall blackness or darkness of an image. It also controls the quantity of x-rays produced. The Reciprocity Law characterizes the relationship within $mAs$, stating that different combinations of mA and time that result in the same total $mAs$ will produce the same exposure ($mA \times \text{time} = mAs$). As a general rule for technique adjustments, $mAs$ is typically cut in half or doubled. To see a visible difference in an image, a minimum change of approximately $30\%$ in $mAs$ is required. It is important to note that $mAs$ does NOT control contrast.

Essential Definitions and Physics Concepts in Radiography

Physics is defined as the study of matter, energy, and how they interact. Energy is the ability to do work, and in the context of radiography, electrical energy is converted into x-ray energy and heat energy. Kinetic energy is the energy of motion, specifically referring to the moving electrons within the x-ray tube. Binding energy is the energy required to remove an electron from its shell, particularly the inner shells. This energy is the force that holds an electron in orbit around the nucleus, and inner-shell electrons possess stronger binding energy because they are closer to the nucleus. During x-ray production, electrons must have enough energy to overcome this binding energy.

Thermionic emission is the process of boiling off electrons from a heated filament. When the filament reaches a high enough temperature, electrons are released from its surface to form a space charge. Rectification is the conversion of alternating current ($AC$) to direct current ($DC$), ensuring that electrons flow in only one direction from the cathode to the anode. Radiographic density is the overall darkness of an image, whereas recorded detail, also known as spatial resolution, refers to the sharpness and clarity of the structures seen on the image. Penumbra refers to the blurred edges or geometric shadow seen on an image, which increases with larger focal spot sizes or poor geometry.

The X-ray Tube, Equipment, and Componentry

The x-ray tube consists of two primary sides: the anode and the cathode. The anode carries a positive ($+$) charge and is the target where electrons strike to produce x-rays. The cathode is the negative ($-$) side and contains the filament, which is the source of electrons. The filament is typically made of tungsten due to its high melting point, efficient electron emission, and durability under heat. Some filaments may use tungsten-thorium, while mammography often utilizes molybdenum and rhodium to produce x-ray energies better suited for breast imaging. Most diagnostic x-ray tubes contain two filaments: a small filament for lower mA stations and better detail, and a large filament for higher mA stations and thicker anatomy.

The focusing cup is negatively charged to repel electrons and focus them into a narrow stream toward the anode target. The space charge effect refers to the cloud of electrons that forms around the filament. The line focus principle is a result of the electron beam striking an angled anode target; this angling makes the effective focal spot (the beam projected toward the patient) smaller than the actual focal spot (where electrons strike the target). This improves recorded detail while allowing for better heat dissipation. The anode heel effect describes why radiation intensity is greater on the cathode side of the tube; x-rays traveling toward the anode side must pass through more target material, decreasing their intensity. Therefore, thicker body parts should be positioned toward the cathode side.

Additional clinical equipment includes the AEC (Automatic Exposure Control), which terminates the exposure automatically once enough radiation reaches the detector to prevent over- or underexposure. The collimator is a device used to control the radiation field size, limiting the beam to the area of interest to reduce scatter and patient dose. Grids are used to absorb scatter radiation before it reaches the image receptor to improve contrast. Grids are located behind the patient but in front of the image receptor. The vertical Bucky is a wall stand used for upright exams like chest x-rays. Timers vary in accuracy: synchronous timers are inaccurate for times less than $1/60\,\text{sec}$ because they use $AC$ cycles, while electronic timers are more precise, measuring in milliseconds. An $mAs$ timer terminates the exposure once a pre-selected $mAs$ value is reached.

Atomic Structure and the Nature of Radiation

All matter is composed of atoms, which are the smallest units of matter and the basic building blocks of everything. Atoms consist of protons, neutrons, and electrons. An atom that loses or gains an electron becomes an ion. The periodic table is arranged by atomic number, which represents the number of protons in the nucleus. Mass refers to the amount of matter in an object.

X-rays are a form of ionizing electromagnetic radiation. They possess three primary properties: they have no mass, carry no charge, and travel in straight lines. Like all electromagnetic radiation, x-rays travel at the speed of light, which is approximately $186,000\,\text{miles/second}$ or $3 \times 10^8\,\text{m/s}$. Radiation is classified into two types: ionizing (capable of removing electrons from atoms) and non-ionizing (lacking sufficient energy to ionize atoms). Natural background radiation comes from the sun, stars, and earth. Gamma rays are high-energy electromagnetic radiation resulting from radioactive decay; while similar to x-rays, gamma rays originate in the nucleus, whereas x-rays are produced outside the nucleus.

Modern and Historical Circuitry

The x-ray circuit is divided into three sections: the primary circuit, the secondary circuit, and the filament circuit. The autotransformer is located in the primary circuit (the low-voltage or control side), which also houses the circuit breaker, control panel, line compensator, and kVp meter. The filament circuit delivers low voltage and high current to the filament to facilitate thermionic emission, utilizing a step-down transformer. In the United States, electricity is supplied as alternating current ($AC$), but x-ray production requires direct current ($DC$). High-frequency generators are the most efficient because they provide smoother, more constant voltage with minimal voltage ripple. Half-wave rectification is the least efficient method as it only utilizes part of the electrical cycle. Historically, early x-ray tubes were gas-filled, which were less efficient than modern vacuum tubes.

X-ray Production at the Anode and Interactions with Matter

X-ray production at the anode is highly inefficient, resulting in $99\%$ heat and only $1\%$ x-rays. There are two main production processes: Bremsstrahlung and Characteristic radiation. Bremsstrahlung, or "braking radiation," occurs when a fast-moving electron is slowed or deflected by the nucleus of a target atom, converting lost kinetic energy into x-ray photons. Characteristic radiation occurs when an inner-shell electron is ejected from the anode target, and an outer-shell electron drops into the vacancy; the difference in binding energy is released as an x-ray photon.

When x-rays interact with matter (the patient), several effects occur:

• Coherent Scattering (Classical or Thompson Scattering): The photon causes atom vibration but no ionization occurs; it is excitation without ionization.
• Compton Effect: The x-ray photon interacts with an outer-shell electron, ejecting it and causing scatter. This increases occupational exposure and decreases image quality.
• Photoelectric Effect: The x-ray photon is completely absorbed by an inner-shell electron in the patient, which is then ejected. This contributes significantly to image contrast. Note that students often confuse this with Characteristic radiation; Characteristic happens at the anode, while Photoelectric happens in the patient.
• Pair Production: A high-energy x-ray ($1.02\,\text{MeV}$ or higher) interacts with the nucleus to produce two particles: one electron and one positron. This occurs in radiation therapy, not diagnostic radiography.
• Photodisintegration: An extremely high-energy interaction where the nucleus is struck and broken apart; this is not part of routine diagnostic imaging.

Image Quality and Geometric Factors

The most important factor for recorded detail is focal spot size; a small focal spot size reduces penumbra and blur. Distance also plays a role: increasing the SID (Source-to-Image Distance) from $40$ to $72\,\text{inches}$ makes the image sharper and less magnified. The geometric properties of an image include recorded detail, magnification, and distortion. Distortion is caused by the improper alignment of the x-ray tube, the body part, and the image receptor. Photographic properties include density and contrast. The Inverse Square Law states that radiation intensity is inversely proportional to the square of the distance; as distance increases, intensity decreases rapidly. To avoid damaging the equipment, technologists calculate heat units based on kVp, mA, time, and generator factors to prevent overloading and cracking the anode.

Positioning, Clinical Practice, and History

Wilhelm Conrad Röntgen discovered x-rays in $1895$. In a clinical setting, the most important person is the patient, and all decisions should prioritize their care, safety, and comfort. Technologists use calipers to measure body part thickness to determine correct exposure techniques. Special positions include the Trendelenburg position, where the patient's head is lower than their feet. Cassettes should be stored upright (vertically) in a clean, dry place. Professional organizations such as the American Society of Radiologic Technologists (ASRT) oversee the standards for technologists.

Questions & Discussion

1. What is the difference between characteristic radiation and photoelectric effect?
Answer: Characteristic radiation happens at the anode target during x-ray production. Photoelectric effect happens in the patient when the x-ray is absorbed by an inner-shell electron.

2. Why does a small focal spot improve detail?
Answer: Because it reduces penumbra and geometric blur.

3. Why is tungsten used in the filament?
Answer: Because it withstands high temperatures and releases electrons efficiently.

4. Why do we use grids?
Answer: To absorb scatter radiation and improve image contrast.

5. Why is the cathode side more intense?
Answer: Because of the anode heel effect.